The present invention relates to a device for the detection and engagement of enemy helicopters of the type which includes an infrared (IR) search head that is deployed above a helicopter, for example by means of a parachute. The search head contains an infrared sensing unit which detects infrared radiation that is emitted upwardly by the turbine of the helicopter's engine and modulated by the rotor blades. The offset angle of the helicopter, relative to the optical axis of the search head, is determined and used to control combat measures. Such a device is known from published German patent application No. 29 07 249.
In the known devices of this type, a pyroelectric detector or a quadrant detector is used as the sensor. These special detectors are advantageous, on the one hand, because they enable distinctions to be made between the IR radiation coming from the helicopter and the IR radiation from another IR radiation source located in the field of vision of the search head. Specifically, such a distinction can be made because the IR radiation modulated by the rotor blades of the helicopter leads to electrical pulse sequences, i.e., an a.c. component while the IR radiation from the other radiation source leads to a steady or d.c. electrical signal, so that a signal analysis unit only needs to perform analysis of the pulse sequence signals. On the other hand, these detectors are required to determine the direction (azimuth) of the incoming modulated IR radiation and thus the offset angle of the helicopter in order to be able to engage it with corresponding combat measures. While it is possible, with the help of the pyroelectric detector or a quadrant detector, to make a distincition in the signal analysis unit between modulated and unmodulated IR radiation, the sensitivity is too low to be able to measure the helicopter radiation. Pyroelectric detectors, for example, based on barium/strontium/Niobate, have a detection capacity of only D*.apprxeq.10.sup.7 -10.sup.8 cm W.sup.-1 Hz.sup.1/2. Beyond that, it is impossible to determine the type of of helicopter which, as will be explained later, causes a modulation frequency of predetermined magnitude.
Another disadvantage of the known device consists of the fact that both the pyroelectric detector and the quadrant detector only provide a very inaccurate determination of the offset angle, which makes it very difficult to take engagement measures. For example, one cannot perform any precise terminal phase guidance of a missile. In the case of pyroelectric detectors it is hardly possible to obtain a finer gradation of the sensor. While the quadrant detectors could be subdivided into a number of sectors larger that 4, they still do not provide a gradation in the radial direction of the sensor A "fine gradation", and thus a more exact determination of the offset angle of the target helicopter would, of course, be possible with a so-called heat image camera, but it does not provide a distinction between modulated and unmodulated radiation. This is because the individual sensor points are scanned by the signal analysis unit on a line-by-line basis, so that one cannot recognize whether the IR radiation incident upon the individual sensor point is or is not timed. Thus, when a heat image camera is used, there would be a great danger of spotting the wrong targets.